Method of controlling the temperature of molten ferrous metal

ABSTRACT

A method of controlling the temperature of molten ferrous metal poured from a bottom pour transfer ladle over an extended period of time. A thick layer of molten slag is placed on the surface of the molten metal to suppress the formation of convection currents in the ladle. A formula provides the relationship between pouring time and the minimum thickness of the molten slag layer required to obtain a substantially constant temperature of the metal discharged from the ladle.

United States Patent Joseph WsHlinka;

Andrew P. Smith, both of Bethlehem; Andrew J. Slabikosky, Allentown, all of Pa. 40,468

May 18, 1970 Dec. 28, 1971 Bethlehem Steel Corporation [72] Inventors [21 Appl. No. [22] Filed [45] Patented 73] Assignee [54] METHOD OF CONTROLLING THE TEMPERATURE OF MOLTEN FERROUS METAL 7 Claims, 4 Drawing Figs.

[52] US. Cl 164/82, 164/133, 75/53, 75/93, 75/96 [51] lnt.Cl B22d11/l0, C2lc 7/00 [50] Field of Search 164/82,

[56] References Cited UNITED STATES PATENTS 2,493,394 1/1950 Dunn et al. 75/50 X 3,465,8l l 9/l969 Castelet....'. 164/281 Primary Examiner- R. Spencer Annear Attorney-John l. lverson ABSTRACT: A method ofcontrolling the temperature of molten ferrous metal poured from a bottom pour transfer ladle over an extended period of time. A thick layer of molten slag is placed on the surface of the molten metal to suppress the formation of convection currents in the ladle. A formula provides the relationship between pouring time and the minimum thickness of the molten slag layer required to obtain a substantially constant temperature of the metal discharged from the ladle.

METHOD OF CONTROLLING THE TEMPERATURE OF MOLTEN FERROUS METAL BACKGROUND OF THE INVENTION This invention relates to the transfer and pouring of molten ferrous metals. It relates especially to the pouring of molten iron and steel from a refractory lined, bottom-pour transfer ladle.

The manufacture of iron and steel involves many operations. Some of these require molten metal to be held in or transferred in a refractory-lined, bottom-pour transfer ladle. For example, in the manufacture of steel, molten steel is usually tapped from a refining furnace directly into a transfer ladle. The ladle of molten steel is then carried to another location where the steel is either further processed or teemed through a nozzle in the bottom of the ladle into ingot molds or into a continuous casting machine. The flow of the "molten metal through the nozzle is controlled by an internal refractory stopper rod or by an external gate valve.

It is well known by steelmakers that the temperature of "the steel being teemed has a significant effect on the-quality of the ingot or continuous cast product. Since normally there is no convenient way to reheat large amountsof molten ste'el after it has left the refining furnace, the steelmaker must estimate the drop in temperature which will occur in the steel during the various processing and transfer operations and compensate for thisdrop in temperature by starting the teeming operation at a higher than normal temperature so that at the endofthe teeming operation the temperature 'of the 'molten'steel falls within a desired range.

It is also'well known that the temperature of molten steel poured from a bottom-pour transfer ladle will vary considerably during the course of the pouring operation. As a result the temperature of the steel teemed from a ladle into a plurality of ingots will vary; i;e. the temperature of the steel of the first few ingots may be considerably different than the temperature of the steel teemed into the last few ingots. As a result of this variance in temperature the cast product from a single heat of steel may vary considerably in quality.

A uniform pouring temperature-or one falling within a narrow range is very important in the continuous casting of steel, if one wishes to produce a sound, high-quality uniform product. In addition, some continuous casting installations require the molten steel to be in*the ladle for much longer periods of time than was'previously customary with ingotteeming practices. The additional pouring tim'e-leads'not only to greater temperature variation but also results in the steel occasionally freezing while still'i'na tundish min a ladle. Some steelmakers have tried to eliminate this freezing problem by pouring steel from a'ladleintoa tundishequipped with heating elements which serve to heat the steel before it enters a continuous casting mold. However, heated tundishes have not been very successful in overcoming freezing problems.

When molten'iron or steel is held in-a bottom-pour refractory-lined transfer ladle; heat is lostfrom'the molten metal by radiation and convection from its exposed top surface'and by conduction tothe refractory walls and shell. Steelmakershave attempted-to reduce the amount of these heat losses-by'placing an'insulator on the top surface of the molten metal, and sometimes by preheating the ladle lining before pouring the molten metal into'theladle. 7

Most steelmakers haveused-a relatively thin (less than 6 inch) layer of molten furnace slag as the insulator because of its low cost and availability. However, other insulators, such as vermiculite, have also been used. Theoretical laboratory studies (Transactions of the Metallurgical Society of AIME, Volume 242, June 1968, Page 963, FIG. have shown that a relatively thin layer of molten furnace slag, only 2-3 inches thick, placed on the surface of the steel in a ladle will act as a near perfect insulator. These studies show that a thicker layer of slag has very little 'effecfon the total amount of heat lost by a ladle of molten 'steel. With the thin slag lay'er insulator on top of the steel practically' all of the heat lost by the steel is lost through the ladle 'wallsby conduction.

With a thin molten slag cover, the steel in a ladle closest to the slag-metal interface will cool, thereby becoming dense. The cooling metal increases in density and sinks, and descends towards the bottom of the ladle. This downward movement of the cooling metal sets up convection currents in the ladle of steel, with the colder layers of steel being carried down and the hotter layers moving up. This action causes a mixing of the molten steel and results in temperature homogenization. This phenomenon is illustrated schematically by FIG. 1 in which is shown a ladle 1 partially filled with molten steel 2. On top of the molten steel is a thin (less than 6 inch) layer of molten slag 3. The arrows shown illustrate the general pattern set up in the steel of the convection currents which produce mixing. As a result of this mixing and the general cooling of the steel with time, the temperature of the steel being poured from the n02- zle in the bottom of the ladle will decrease. Depending on the pouring time and other factors, such a decrease in temperature could be as much as 100 F.

The broken line in FIG. 4 illustrates the temperature of molten steel being discharged into a tundish from a 7k-ton bottom pour transfer ladle using a relatively thin 2-inch insulating cover of molten furnace slag. The 2-inch thickness of this cover is conventional practice for a ladle of this size. The

variation of steel temperatures in the tundish is directly related to the temperature of the stream being poured into the tundish. The initial pertubations in the temperature of the steel in the tundish during the first few minutes is caused both by the colder steel which was closest to the bottom and first discharged from the ladle and by the early higher heat losses to the tundish. After several minutes the temperature of the steel peaks'as a result of the mixing by the convection currents and then gradually decreases. In this example of a conventional pouring practice there was almost an F. variation in temperature of the steel being poured into the tundish during a 12-minute pouring operation.

SUMMARY OF THE INVENTION It is therefore an object of this invention to provide a method of controlling the temperature of molten ferrous metal poured from bottom-pour tran'sfer'ladles or the like over an'extended period of time.

It is a further objectof this invention to provide a method of teeming large'heats of 'molten steel ingot molds into a continuous ca'stingmachine'with little or no drop in temperature.

Other and'further objects of this invention will become apparent from the following description and claims.

According to the present invention'the foregoing objects are attained by placing a substantially thicker than normal layer of molten slag on the surface of molten metal contained in a 'bottom-pour'transfer ladle. The minimum thickness of the slag layer required for my invention, expressed in inches, is equal to or greater than the product of 0.085 T+5.5, where T is the total time, expressed in minutes, necessary to pour the metal from the ladle.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 are'cross-sectional views of a bottom pour ladle of molten steel which schematically illustrate the convection currents in'thesteel accordingto the pouring practices of the prior art-and of this invention.

FIG. 3 is a graph to illustrate the relationship between total pouring time and the minimum thickness of the molten slag layer required to obtain a substantially constant stream temperature.

FIG. 4 is a graph showing temperatures of molten steel poured into a tundish from 7'r-ton ladles using the pouring practices of the prior art and of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENT In accordance with this invention the temperature of the steel be'ing'discharged from the ladle can be held substantially constant for periods up to 2 hours or more by placing a relatively thick molten slag layer on the'surface of the steel in a ladle.

As pointed out before, a thick layer of slag will not have any significantly greater effect in preventing total heat losses from molten steel than does a thin slag layer. However, a thick layer of slag effectively suppresses local heat losses at the slag-metal interface and thus prevents the formation of convection currents in the central portion of a ladle of steel. This is shown schematically in FIG. 2, in which is shown ladle 1 partially filled with molten steel 2, which is covered with a layer of molten slag at least 6 inches thick. Slag layer 3 does not affect the descending currents at the sidewalls but does effectively prevent a mixing of the steel 2 in the center of the ladle 1.

As a result of model studies we have found that the sidewall currents under the thick layer of slag act as a balanced heat pump by bringing steel to the ladle nozzle at a substantially constant temperature. An explanation for this phenomenon is that in the early stages of a pouring operation the temperature of the steel which joins the descending sidewall currents is high but the heat loss to the colder ladle walls is great. Early in the pouring operation the sidewall streams of steel lose more heat to the ladle walls but from a higher temperature level as compared to later temperatures. Later in the pouring operation, the temperature of the steel which joins the descending sidewall currents is lower but, because the ladle walls are now hotter, the steel loses less heat. This balancing requires that the bulk of steel in central part of the ladle remain essentially stagnant. This stagnation is accomplished by providing a layer of slag at least 6 inches thick.

The solid line in FIG. 4 illustrates the temperature of the stream of molten steel being discharged into a tundish from a 7'r-t0n bottom pour transfer ladle using a thick layer of slag according to this invention. In this example the slag layer was 6 inches. The initial temperature for this example was about 35 higher than the previously described example shown by the dashed line. The solid temperature curve shows the same initial drop in temperature during the first minutes and a subsequent rise in temperature. However, as a result of the thick slag layer of this invention the temperature thereafter remained substantially constant during the remainder of the 12-minute pouring operation.

We have found from both laboratory and plant studies that a relation exists between the length of the pouring operation and the minimum thickness of molten slag cover required to produce the substantially constant temperature of the stream of steel being discharged from the ladle. This relationship is illustrated by the straight line in FIG. 2. Thus, for example, if the total time required to pour the ladle of the steel is 80 minutes, according to our invention it would be necessary to have on the steel in the ladle a molten slag layer at least 12% inches thick to obtain a substantially constant temperature over the 80 minutes.

This relationship can be expressed by the following formula:

Minimum Slag Layer Thickness 0.085XT+5.5 where the minimum slag layer thickness is expressed in inches and T is the total pouring time expressed in minutes.

Slag layer thicknesses substantially greater than the minimum as determined above can be used according to this invention but this may produce an increase in the temperature of the steel during the latter stages of the pouring operation.

The thickness of the layer of slag is measured aft :r the foaming of the slag has subsided. Such measurements can be made, as is well known to those skilled in the art, by using known reference points on the ladle lining or stopper rod.

As used in this specification the term substantially constant temperature" means that the temperature of the stream of metal being discharged from the nozzle of the ladle does not vary during the entire pouring operation more than 30 F. after the initial pertubation in temperature which occur during the first few minutes of the pouring operations, as shown in FIG. 4.

This invention may be used for the pouring of molten iron as well as steel and thus could be used in iron foundries for the casting of iron shapes. The invention is especially useful for the continuous casting of steel since it provides a method of introducin the molten steel into a tundish or the continuous casting mo d at a substantially constant temperature over an extended period of time.

We claim:

1. A method of maintaining a substantially constant temperature of molten ferrous metal being poured from a bottompour ladle into a receptacle over an extended period of time comprising:

a. pouring the molten ferrous metal into said ladle,

b. placing a layer of molten slag obtained from the furnace in which the molten ferrous metal was melted and refined on the surface of said metal in said ladle, the thickness of said slag expressed in inches, being at least equal to the value of 0.085 XT +5.5, where T is the total pouring time, expressed in minutes, and

c. pouring said molten metal from the bottom of said ladle into said receptacle.

2. The method of claim 1 in which the molten ferrous metal is steel.

3. The method of claim 1 in which the receptacle is a tundish.

4. The method of claim 1 in which the receptacle is a mold.

5. In the continuous casting of steel, a method of maintaining a substantially constant temperature of molten steel being poured into the continuous casting machine comprising:

a. tapping the molten steel into a bottom pour transfer ladle,

b. placing a layer of molten slag on the surface of said steel in said ladle, the thickness of said slag layer, expressed in inches, being equal to or greater than the value of 0.085 XT +5.5, where T is the total pouring time, expressed in minutes,

c. pouring said steel from the bottom of said ladle into a receptacle forming a part of said continuous casting machine.

6. The method of claim 4 in which the receptacle is an openended mold.

7. The method of claim 4 in which the receptacle is a tundish. 

1. A method of of maintaining a substantially constant temperature of molten ferrous metal being poured from a bottompour ladle into a receptacle over an extended period of time comprising: a. pouring the molten ferrous metal into said ladle, b. placing a layer of molten slag obtained from the furnace in which the molten ferrous metal was melted and refined on the surface of said metal in said ladle, the thickness of said slag expressed in inches, being at least equal to the value of 0.085 X T plus 5.5, where T is the total pouring time, expressed in minutes, and c. pouring said molten metal from the bottom of said ladle into said receptacle.
 2. The method of claim 1 in which the molten ferrous metal is steel.
 3. The method of claim 1 in which the receptacle is a tundish.
 4. The method of claim 1 in which the receptacle is a mold.
 5. In the continuous casting of steel, a method of of maintaining a substantially constant temperature of molten steel being poured into the continuous casting machine comprising: a. tapping the molten steel into a bottom pour transfer ladle, b. placing a layer of molten slag on the surface of said steel in said ladle, the thickness of said slag layer, expressed in inches, being equal to or greater than the value of 0.085 X T plus 5.5, where T is the total pouring time, expressed in minutes, c. pouring said steel from the bottom of said ladle into a receptacle forming a part of said continuous casting machine.
 6. The method of claim 4 in which the receptacle is an open-ended mold.
 7. The method of claim 4 in which the receptacle is a tundish. 